you are giving them too much water. the roots are suffocating they need air. i have seen this a 1000 times. the plant needs a good wet dry cycle. if you want to solve fast repot into a new mix and only lightly mist the new soil then leave iit alone for a week.
I have gotta agree.
Too much water and not enough air circulation is causing your problems.
Just looking at the plant gives the mental image of a sponge that needs wrung out.